The Best Solution for ESD of High Resolution Panels

by Man-Jyun Fang

In recent years, panel manufacturers have focused on the development of high-quality panels, and each of them has invested high-end panel design for differentiation; product types include notebooks, TVs, displays and AIO (all-in-one) computers. However, with the rise of high-end panels, many new problems have emerged.

● Transmission principle of panel signal

Let's start with panel architecture. Some say that liquid crystal is the heart of panel, and then the time controller (T-CON) is the brain of it. As shown in figure 1, T-CON receives image data from GPU (Graphics Processing Unit) of TV or notebook, convert the format for source driver and generates control signals to Gate driver and Source IC then outputs voltage to control liquid crystal arrangement direction for producing screen. Transmission interfaces of T-CON receiver (Rx) and transmitter (Tx) are changed with various resolution needs.

Figure 1 Panel architecture schematic diagram

● The evolution and problem derivation of T-CON transmitter interface

Low-voltage Differential Signaling (LVDS) is the most basic type among T-CON Rx transmission interfaces. Since the introduction of this interface by Apple and National Semiconductor in 1994, it has been in use ever since. However, with the improvement of specifications in TV size, screen resolution and frequency, LVDS is unable to meet the new generation 8K transmission requirements (as shown in table 1). Therefore, the use of following two high-speed interfaces in panel has increased year by year: the V-by-One technology of Japan Thine and eDP (VESA Embedded Display Port) technology jointly introduced by Intel/AMD/Nivida and Apple. 

Table 1: Comparison of V-by-One ®US and existing transmission interfaces

Electrostatic Discharge (ESD) is a natural phenomenon that cannot be avoided in the environment. Panel has multiple processes, and there are many opportunities to contact with other objects during alignment and coating. The film must also be attached and detached when assembly; they will cause frictional electricity generation. Also, as the panel resolution increased; more ports are needed for multiple circuit board connections, increasing the total number of plugging and unplugging during assembly and testing. ESD is therefore generated that damages IC and results in screen abnormality.

For the damage caused by plugging/unplugging, please refer to discussion of hot plug proposed by Dr. Jiang Xinqin in ESD Protection Design Technology 1. Use Direct pin injection test method to apply ESD to I/O directly to simulate actual hot plug.

● 8K new generation transmission ESD solution – receiver       

Regarding above issues, ESD protection device can be placed on external connection port of T-CON logic board as the protection of front line. At this point, there are two main points to be aware of when selecting protection device: first, the parasitic capacitance of ESD protection device should be low enough, the lower limit of parasitic capacitance of V-by-One ®HS and V-by-One ®US are different. Second, for Direct pin injection test, the clamping voltage of ESD protection device during ESD event must be low enough to prevent error in transmission process and damage to the T-CON. 

Amazing Microelectronic provides the most effective ESD protection device - TVS (Transient Voltage Suppressor device). Compare with other ESD protection device, the low clamping voltage characteristics of TVS make protection effectiveness reached Class A. LVDS and V-by-One®HS have a general parasitc capacitance of 0.45 pF TVS[AZ1043-08F], its layout is shown in figure 2. The feature of Feed-through enables convenient layout and easier to achieve impedance matching. As for V-by-One®US that will be on 8K stage, because the transmission speed of which reaches 16Gbps of a differential pair, it is recommended to use AZ111S-08F TVS that is able to pass the highest test level of IEC 61000-4-2: Contact 8kV/air 15kV as shown in figure 3; its extremely low parasitic capacitance (0.1pF) meets various high-speed applications in the future.

Figure 2: AZ1043-04F is applied to V-by-One®HS

Figure 3: AZ111S-08F is applied to V-by-One®US

● 8K new generation transmission ESD solution - transmitter and other ports       

The ESD protection device must be located external connection port of T-CON logic board as mentioned above, the Tx end of T-CON is also included (as shown in figure 1). At present, the most common transmission interface is Mini-LVDS. When ESD enters from its port, Mini-LVDS may be damaged, resulting in vertical bright line on panel. Therefore, it is recommended to install TVS on Mini-LVDS port to prevent data transmission from being affected by ESD or even damaging T-CON permanently. The transmission speed of Mini-LVDS is 331Mbps, so a 0.45pF AZ1043-08F can be added on Mini-LVDS port for protection. In addition, figure 1 also shows that the interface of panel also includes I2C, control signals and PWM signals which adjusting backlight control. These low speed signals or other power cables do not require strict capacitance value of TVS. After selecting TVS with proper capacitance value, then make option according to system maximum operation voltage.       

Looking at the structure of the entire panel, it includes many large-area insulating materials (such as glass). It accumulates static electricity in the front-end process which may bombard internal IC easily when ESD event occurs during assembly. With the high-ordering of T-CON, micro-reproduction of chip process is easier to be damaged due to ESD. There are many reasons that cause much higher defective rate for panel assembler in comparison with electronic products of other fields. TVS plays a key role in panel manufacture protection now. A good TVS is able to avoid expensive main chip being burnt and production line shut down during panel manufacture process.